As is well known, government regulations require vehicles equipped with internal combustion engines not to emit certain specific gaseous pollutants or emissions produced by the engine beyond certain set emission threshold levels. Typically, the vehicle cannot exceed the emission thresholds when operated pursuant to a specified driving cycle such as that set forth in a FTP (Federal Test Procedure). The FTP requires that the vehicle be operated at various acceleration/deceleration modes as well as at steady state or constant velocity at various specified speeds in a standardized drive cycle. In the course of the drive cycle, the engine emits varying concentrations of specific gaseous emissions. Current laws in California, in other states, and around the world require that emission reduction equipment incorporated on a vehicle be continuously monitored by on-board diagnostic (OBD) systems. The function of these OBD systems is to report and set fault codes or alarm signals when the emission control devices no longer meet the mandated emission levels. One of the systems to be monitored is the catalytic converter which in current automotive applications is used to simultaneously reduce the levels of carbon monoxide, oxides of nitrogen (NO.sub.x) and un-burnt hydrocarbons (HC) in the exhaust gases. This invention relates to such a system.
Typical emission systems currently used today employ a TWC catalytic converter (three-way catalytic converter--NO.sub.x, hydrocarbons and oxides, i.e., CO). TWCs store oxygen when the engine operates lean and release stored oxygen when the engine operates rich to combust gaseous emissions such as hydrocarbons or carbon monoxide. As the TWC catalyst ages, its ability to store oxygen diminishes and the efficiency of the catalytic converter decreases. Based on this fact, current OBDs in use today comprise an exhaust gas oxygen sensor (EGO) placed upstream of the catalyst and an EGO placed downstream of the catalyst to provide some indication of the capability of the catalyst to store oxygen. This provides an estimate of a direct measurement of the oxygen storage capacity of the catalyst. Through calibration, this measurement of an estimate of the oxygen storage capacity of the catalyst can be related to the ability of the catalytic converter to convert the regulated exhaust gas emissions, i.e., the conversion efficiency of the catalytic converter.
It is also known to conventionally control the air/fuel ratio during certain portions of the driving cycle to generate rich/lean exhaust gas mixtures to measure oxygen storage capacity of the TWC. Still further, a number of before and after oxygen sensing techniques have been employed to sense when the deterioration of the catalyst has progressed to an alarm actuating condition. Reference can be had to Kayanuma U.S. Pat. No. 5,283,383; Seki U.S. Pat. No. 5,636,514; and Carnevale et al. U.S. Pat. No. 5,697,214. In at least one instance, as indicated by Ito et al. U.S. Pat. No. 5,357,750, the observation is made that when the engine is forced to operate lean, certain correlations can be made to conversion rates for nitrous oxide and when the engine is forced to operate at a richer fuel ratio, certain correlations or relationships can be made to the conversion efficiencies for hydrocarbon and carbon monoxide vis-a-vis oxygen and A/F ratio sensors upstream and downstream of the converter.
Systems that use EGO sensors and techniques based on such systems are inherently flawed because they monitor the oxygen storage capacity of the catalyst and not the gaseous emissions which must be controlled. Additionally, monitoring techniques based on catalyst deterioration by mechanisms not used to initially calibrate the catalytic converter, may result in a false identification of the status of the emission system giving erroneous fail signals or failing to give a failed signal. Attempts to address the problem are discussed in U.S. Pat. Nos. 5,283,383 and 5,636,514.
Perhaps the most substantial problems with current EGO sensor techniques using oxygen storage to either measure emissions or catalyst deficiencies are the restrictions imposed on the catalyst design and engine management strategies.
From the catalyst design viewpoint, the factors leading to the deterioration of the oxygen storage capacity of the catalytic converter and the precious metal surface area (which directly accounts for HC, CO and NO.sub.x conversion) must be roughly matched with one another so that monitoring the catalyst (which really amounts to monitoring the precious metal surface area) can be accomplished by monitoring the oxygen storage capacity. Apart from any discussion relating to restrictions imposed on the design of the catalytic converter to achieve the desired match of precious metal surface area with OSC, emission regulations now coming into effect require that the vehicle control the emissions when the engine is cold or on startup and not only when the vehicle has reached its operating temperature. This requirement is met by the use of close-coupled, light/off catalysts, which may be void of rare-earth metal oxides. Such catalytic converters lack any oxygen storage function and cannot employ or use oxygen sensors to measure oxygen storage capacity.
From the engine control standpoint, and specifically to obtain good fuel control, it is favorable to increase the air to fuel perturbation rates to 10 Hz or greater to enable optimal performance of three-way catalyst. At such rapid cycling rate, it is impossible to discern meaningful variations in oxygen storage capability.
In fact, the oxygen storage capacity of the TWC has been demonstrated to poorly correlate with hydrocarbon conversion efficiencies. See J. S. Hepburn and H. S. Gandhi, "The Relationship Between Catalyst Hydrocarbon Conversion Efficiency and Oxygen Storage Capacity", SAE paper 920831, 1992 and G. B. Fischer, J. R. Theis, M. V. Casarella and S. T. Mahan, "The Role of Ceria in Automotive Exhaust Catalysis and OBD-II Catalyst Monitoring", SAE paper 931034, 1993.
Because of all the limitations discussed above, it is generally acknowledged that dual EGO approaches develop signals with poor resolution with only the ability to determine gross changes in the catalyst conversion efficiency. The deficiencies in the OSC monitoring systems discussed above have been recognized in the prior art and to circumvent these deficiencies it is known to attempt to directly measure the emissions to determine whether the vehicle is or is not in compliance with standards. Specifically, it is known to use gas sensors of the calorimetric type to measure gaseous emissions of the vehicle. That is, emissions react with oxygen or oxidants to release heat and the heat released is measured by the calorimetric sensor. The heat sensed by the calorimetric sensor is then correlated to the emissions producing the heat release. Reference can be had to U.S. Pat. Nos. 5,444,974 to Beck et al., dated Aug. 29, 1995 and 5,451,371 to Zanini-Fisher et al., dated Sep. 19, 1995. In the '974 patent, the calorimetric device is actuated only when the air/fuel ratio is lean so that sufficient oxygen is present to promote the exothermic oxidation reactions. In the system patents which utilize the sensor described in the '371 patent, gas samples upstream and downstream of the sensor are tapped and combusted with oxygen so that rich samples of exhaust gas can be analyzed. The use of such sensors is thus limited.
Such limitations have been somewhat overcome by utilization of a solid electrolyte to generate oxygen transfer in combination with a diffusion membrane to provide, in effect, an "oxygen pump" within the calorimetric sensor so that rich combustibles in rich concentrations can be reacted. See, for example, U.S. Pat. No. 5,505,836 to Friese et al. dated Apr. 9, 1996. More recently, calorimetric sensors have been significantly improved by the designs disclosed in U.S. patent application Ser. No. 08/970,837, filed Nov. 14, 1997, entitled "Calorimetric Hydrocarbon Gas Sensor" and Ser. No. 08/970,259, filed Nov. 14, 1997, entitled "Exhaust Gas Sensor" incorporated by reference herein. Calorimetric sensors recently developed by the assignees of the present invention permit direct, dynamic measurement of gaseous emissions by simply inserting only one calorimetric sensor downstream of the catalytic converter which produces better, more consistent and reliable measurements than that capable of being achieved with EGO sensors and OSC management techniques.
It must be appreciated that the calorimetric sensors developed to date are extremely sensitive since they measure minute traces of gaseous emissions generating small heat release quantities. As the emission standards become tighter or more stringent, the sensitivity of the calorimetric sensor has to increase to detect smaller and smaller quantities of heat released by the exothermic oxidation reactions produced by smaller and smaller emissions concentrations. In addition, the sensor has to operate for "cold engine" emission detection which further complicates the problem since exhaust gas temperature for a cold engine is different than that for a hot engine. Simply increasing the gain of the sensor to increase sensor sensitivity produces excessive variations in signal output. While various filtering schemes could be utilized to cancel out some variations, a more inherent problem is present when sensitivity has to be increased to meet ultra low levels of emissions in a dynamic operating environment. More specifically, any change in heat transfer dynamically affects the base reference point or the zero point upon which the sensor measurement is based. Furthermore, the heat transfer resulting from the exhaust is several orders of magnitude larger than that attributed to heat releases of minute emission concentrations. These heat reactions, termed "secondary" only for the purposes of discussion, arise in a dynamic operating environment of the internal combustion engine. Until this invention, such secondary reactions prevented use of a calorimetric sensor to directly measure absolute values of gaseous emissions to determine whether or not such emissions comply with stringent LEV and ULEV regulations.